There are two basic techniques used in data converters. These are the sigma-delta technique and the resistive or capacitive divider techniques. The sigma-delta technique is attractive because it achieves high resolution by high-frequency timing instead of precisely-matched on-chip components such as resistors. In addition, the expertise needed to produce thin-film, laser-trimmed analog components is difficult to obtain; whereas, high-speed digital switching capability used in sigma-delta modulators is commonplace in the semiconductor industry.
A basic sigma-delta modulator used in a data converter receives an input signal which is summed with inverse feedback of the output signal to provide an error signal. The error signal is processed through a filter to provide the output signal. The sigma-delta modulator shapes quantization noise out of the passband, where it can be filtered.
However, as is well known, sigma-delta modulators can create unwanted tones in the passband. These unwanted tones have frequencies which are linearly related to a DC bias of the input signal. The creation of these unwanted tones is worse for small input signals which are close to the signal ground level. Furthermore, during the time when the input signal is close to the ground level, it is easier for a listener to hear the unwanted tones. One generally accepted explanation of why sigma-delta modulators produce these unwanted tones is that the sigma-delta modulator creates pattern noise in order to resolve small incremental changes in the input signal.
Previous methods for eliminating unwanted tones in the passband included using dithering schemes. However, the implementation of a dither scheme for data conversion is complex. Furthermore, a dither signal affects the overall performance of the data converter such that the signal-to-noise ratio of the data converter is reduced.
Accordingly, it would be advantageous to have a sigma-delta converter which reduces or eliminates unwanted tones in the passband caused by pattern noise. It would be of further advantage for the sigma-delta converter to be area and cost efficient.